Apparatus and method for generating preamble sequence in a BWA communication system using OFDM

ABSTRACT

A preamble sequence generating apparatus and method in a BWA communication system using OFDM. An ARM code generator generates at least one ARM code of a different length. A sub-carrier selection and frequency mask unit maps a second number of ARM code components to a second number of sub-carriers among a third number of OFDM sub-carriers distributed equally in a frequency band and maps null components to sub-carriers excluding from the second number of sub-carriers. Here, the second number being the number of (the components of the ARM code) minus (a DC component and a first number of ARM code components set to prevent adjacent channel interference). Then, an IFFT generates a preamble sequence by inverse-fast-Fourier-transforming the sub-carriers received from the sub-carrier and frequency mask unit.

PRIORITY

[0001] This application claims priority to an application entitled“Apparatus and Method for Generating Preamble Sequence in a BWACommunication System Using OFDM” filed in the Korean Industrial PropertyOffice on Aug. 27, 2001 and assigned Serial No. 2001-51900, the contentsof which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates generally to a BWA (BroadbandWireless Access) communication system using OFDM (Orthogonal FrequencyDivision Multiplexing), and in particular, to an apparatus and methodfor generating a preamble sequence using an ARM (Aperiodic RecursiveMultiplex) code.

[0004] 2. Description of the Related Art

[0005] In general, a wireless communication system supports wirelesscommunications and includes Nodes B and UEs (User Equipments). A Node Band a UE use transmission frames to support the wireless communications.For transmission and reception of transmission frames, the Node B andthe UE must mutually acquire synchronization. For this purpose, the NodeB transmits a sync signal to inform the UE of the start of a transmittedframe. The UE then acquires frame timing from the sync signal anddemodulates the frame according to the frame timing. A particularpreamble sequence that is preset between the Node B and the UE is usedas the sync signal. When the preamble sequence is transmitted as a burstsignal, its reception performance depends on aperiodic autocorrelationcharacteristics.

[0006] An OFDM communication system uses a preamble sequence with a lowPAPR (Peak to Average Power Ratio). The Node B transmits to the UE ashort preamble sequence for coarse synchronization concatenated with along preamble sequence for fine frequency synchronization. On theuplink, the UE transmits to the Node B a long preamble sequence for finefrequency synchronization. In the case of an STC (Space Time Code) framedirected from the Node B to the UE, an STC preamble sequence isconfigured with the same structure such that STC preamble-basedsynchronization acquisition and channel estimation is possible in UEsregardless of receiving STC signals or not receiving STC signals.

[0007] Similar to the preamble sequence for frame timing acquisition,there exists a sequence for burst synchronization acquisition. In a BWAcommunication system, for example, using FDD (Frequency DivisionMultiplexing), a plurality of burst slots exist in a frame. Therefore aslot sync signal is used to find the start of the slots, and a midamblesignal exists in each burst slot for burst slot synchronization in theTDD communication system. The slot sync signal or midamble signal isalso transmitted in the form of a preset sequence between the Node B andthe UE, similar to the frame sync signal.

[0008] The OFDM BWA communication system transmits data in atime-multiplexed frame to a plurality of UEs. A frame preamble istransmitted for a predetermined time period from the start of the frameto identify the start of the frame and a burst preamble exists at thestart of data due to intermittent data transmission for the UEs in theframe. To detect the start of the data transmission, the UEs receivedata preambles. To synchronize to the start of data for data reception,the UEs acquire a preamble signal commonly used throughout the system.

[0009] The OFDM communication system employs the same source coding,channel coding, and modulation as non-OFDM communication systems. Whiledata is spread prior to transmission in CDMA (Code DivisionMultiplexing), data is inverse-fast-Fourier-transformed and guardintervals are inserted into the data in OFDM. Accordingly, OFDMtransmits broadband signals using relatively simple hardware. In theOFDM communication system, an RFFT (Inverse Fast Fourier Transformer)outputs a time domain signal for the input of parallel bit/symbolsequences after data modulation. The time domain signal containsmultiplexed signals on sub-carriers divided from a broad band. Multiplemodulations symbols for an OFDM duration are summed by the IFFTprocessing.

[0010] Transmission of IFFT OFDM symbols without any additionalprocessing, however, causes interference between the previous OFDMsymbol and the current OFDM symbol. To eliminate the intersymbolinterference (ISI), guard intervals are inserted. The guard intervalinsertion is achieved by transmitting null data in correspondingpositions, but wrong estimation of the start of an OFDM symbol in areceiver may cause interference between sub-carriers and increase anOFDM symbol decision error rate. Hence as a guard interval, a cyclicprefix or a cyclic postprefix is proposed. The cyclic prefix is to copythe last n/I bits of a time-domain OFDM symbol and insert them in aneffective OFDM symbol. The cyclic postprefix is to copy the first 1/nbits of a time-domain OFDM symbol and insert them in an effective OFDMsymbol. The duplication and disposition of the first or last portion ofan OFDM symbol in a particular OFDM symbol enables acquisition of OFDMsymbol time/frequency synchronization in the receiver.

[0011] Meanwhile, a transmitted signal is distorted during transmissionon a radio channel. The receiver acquires time/frequency synchronizationand channel estimation using a preset preamble signal and demodulatesthe distorted data to symbols in a frequency domain by FFT (Fast FourierTransform). Then the receiver decodes information data by subjecting thedemodulated symbols to channel decoding and source decoding incorrespondence with channel coding used in the transmitter.

[0012] The preamble signal is used for frame timing synchronization,frequency synchronization, and channel estimation in the OFDMcommunication system, although guard intervals and pilot sub-carrierscan also be used for the same purpose. Known symbols are transmitted asa preamble signal at the start of each frame or data burst.Time/frequency/channel estimation information obtained from the preamblesignal is then updated using a guard interval and a pilot sub-carrier indata of the frame.

[0013] The preamble sequence must be generated, taking into account thefollowing parameters.

[0014] (1) The PAPR of OFDM symbols must be low to maximize thetransmission efficiency of a power amplifier in the transmitter. OFDMsymbols in the time domain at the output of an IFFT in the transmittermust have a uniform power distribution. In other words, if symbols witha low cross correlation in the frequency domain are input to the IFFT,the IFFT output has a low PAPR.

[0015] (2) Since the receiver estimates time/frequency information bycross correlation or autocorrelation, OFDM symbols in the time domainmust be repeated. To do so, null data (zeros) are interpolated in afrequency-domain sequence.

[0016] (3) A virtual carrier or a guard band is defined to minimizeadjacent channel interference. In OFDM being a multi-carrier system,null data is inserted in the outermost sub-carriers in the frequencydomain to solve the problem of adjacent channel interference. Theoutermost sub-carriers are the middle indexes among IFFT input indexes.

[0017] Optimum communication performance is achieved only if the abovethree conditions are satisfied when a preamble sequence is generated inthe OFDM communication system.

[0018] A frame structure with the above-described preamble will bedescribed below with reference to FIG. 1. FIG. 1 illustrates thestructure of a frame in a typical BWA communication system.

[0019] Referring to FIG. 1, reference numeral 101 denotes a framepreamble indicating the start of the frame to enable a UE to acquireframe timing synchronization to a Node B. The frame preamble 101 ismodulated by BPSK (Binary Phase Shift Keying) or QPSK (Quadrature PhaseShift Keying). Reference numeral 102 denotes the frame containing realinformation data. The data in the frame 102 is modulated by QPSK, 16QAM(Quadrature Amplitude Modulation), or 64 QAM and then multiplexed inOFDM. N frames are illustrated in FIG. 1. This means that N channelstransmit data following downlink/uplink preambles or STC preambles.

[0020] As described before, a preamble sequence preset between thetransmitter (Node B) and the receiver (UE) is used for frequencysynchronization and channel estimation. In view of the burst property ofthe preamble sequence, it requires a low PAPR and excellent performancein time/frequency synchronization and channel estimation.

[0021]FIG. 5 illustrates a preamble structure in the typical BWAcommunication system. Referring to FIG. 5, a short preamble 501 and longpreambles 502 and 503 are used in the BWA communication system. Theshort preamble 501 is used for coarse synchronization between a Node Band a UE and the long preambles 502 and 503, for fine frequencysynchronization. A downlink preamble being the short preamble 501concatenated to a long preamble is at least 160 samples in duration, andan uplink burst preamble is at least 512 samples in duration. Referencenumeral 504 denotes an STC preamble transmitted to UEs regardless ofwhether they support STC or not, for frequency synchronizationacquisition and channel estimation. Hence a BWA system requires preamblesignals of different lengths and thus requires an apparatus and methodcommonly used to generate the different preamble signals, particularlywith excellent PAPR characteristics, and time/frequency synchronizationand channel estimation performance.

SUMMARY OF THE INVENTION

[0022] It is, therefore, an object of the present invention to providean apparatus and method for generating a preamble sequence in a BWAcommunication system.

[0023] It is another object of the present invention to provide anapparatus and method for simultaneously generating a frame preamblesequence and a burst preamble sequence in a BWA communication system.

[0024] It is a further object of the present invention to provide anapparatus and method for generating a preamble sequence with a minimumPAPR in a BWA communication system.

[0025] To achieve the above and other objects, an ARM code generatorgenerates at least one ARM code of a different length, and a sub-carrierselection and frequency mask unit maps a second number of ARM codecomponents to a second number of sub-carriers among a third number ofOFDM sub-carriers distributed equally in a frequency band and maps nullcomponents to sub-carriers excluded from the second number ofsub-carriers. Here, the second number being the number of the componentsof the ARM code, minus, a DC component and a first number of ARM codecomponents set to prevent adjacent channel interference. Then, an IFFTgenerates a preamble sequence by inverse-fast-Fourier-transforming thesub-carriers received from the sub-carrier and frequency mask unit.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The above and other objects, features and advantages of thepresent invention will become more apparent from the following detaileddescription when taken in conjunction with the accompanying drawings inwhich:

[0027]FIG. 1 illustrates a frame structure in a typical BWAcommunication system;

[0028]FIG. 2 is a block diagram of an MPP (Minimum PAPR Preamble)sequence generating apparatus according to an embodiment of the presentinvention;

[0029]FIG. 3 schematically illustrates the operations of a sub-carrierselector and a frequency mask illustrated in FIG. 2;

[0030]FIG. 4 is a block diagram of an ARM code generator illustrated inFIG. 2;

[0031]FIG. 5 illustrates the structures of preambles in the typical BWAcommunication system;

[0032]FIG. 6 schematically illustrates short preamble sequencegeneration in the MPP sequence generating apparatus illustrated in FIG.2;

[0033]FIG. 7 schematically illustrates long preamble sequence generationin the MPP sequence generating apparatus illustrated in FIG. 2;

[0034]FIG. 8 schematically illustrates STC preamble sequence generationin the MPP sequence generating apparatus illustrated in FIG. 2;

[0035]FIG. 9 illustrates preamble sequences generated from the MPPgenerating apparatus illustrated in FIG. 2;

[0036]FIG. 10 illustrates a frequency mask according to anotherembodiment of the present invention; and

[0037]FIG. 11 illustrates a frequency mask according to a thirdembodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0038] Preferred embodiments of the present invention will be describedherein below with reference to the accompanying drawings. In thefollowing description, well-known functions or constructions are notdescribed in detail since they would obscure the invention inunnecessary detail.

[0039] A preamble sequence proposed in the present invention isapplicable to BWA communication systems of which the standardization isbeing carried out and other communication systems using preamblesequences described above. The preamble sequence is characterized by itslow PAPR and thus referred to as an MPP (Minimum PAPR Preamble)sequence. The present invention provides an MPP sequence generatingapparatus and method in a BWA communication system. As preamblesequences of the present invention for time offset synchronization,frequency offset synchronization, and channel estimation, a shortpreamble or a long preamble, and an STC preamble will be described.

[0040]FIG. 2 is a block diagram of an MPP sequence generating apparatusaccording to an embodiment of the present invention. Referring to FIG.2, the MPP sequence generating apparatus is comprised of an ARM codegenerator 200, a sub-carrier selector 201, a frequency mask 202, an IFFT203, and a preamble assembler 204. The ARM code generator 200 isdisclosed in Korea Patent Application No. 2000-0071092, filed in theU.S. Patent and Trademark Office on Nov. 21, 2001 and assignedapplication Ser. No. 09/990,557, the contents of which are hereinincorporated by reference, and its operation will be described referringto the patent application.

[0041]FIG. 4 is a block diagram of the ARM code generator 200. Referringto FIG. 4, the ARM code generator 200 generates a complex ARM code oflength 16. A multiplier 410 receives a signal +1 and −1 or −1 and +1 ina sequence of alternating +1s and −1s from a signal generator 420.Regardless of whether +1 or −1 is first received, an ARM code producedfrom the ARM code generator 200 has the same characteristics. Uponreceipt of one of all binary combinations of two bits +1 & +1, +1 & −1,−1 & +1, or −1 & −1, the multiplier 410 multiplies the bits by thesignal from the signal generator 420.

[0042] A first multiplexer (MUX 1) 400 multiplexes the input signal andthe output of the multiplier 410 in time and outputs a 4-bit sequence. Amultiplier 412 multiplies the 4-bit sequence by a 4-bit signal receivedfrom a signal generator 422 and a second MUX (MUX 2) 402 multiplexes theoutputs of the first MUX 400 and the multiplier 412 in time and outputsan 8-bit sequence. A multiplier 414 multiplies the 8-bit sequence outputby the second Max 402 by a 8-bit signal received from a signal generator424, +1, −1, +1, −1, +1, −1, +1, −1 or −1, +1, −1, +1, −1, +1, −1, +1. Athird MUX (MUX 3) 404 multiplexes the outputs of the second MUX 402 andthe multiplier 414 in time and outputs a 16-bit sequence. The 16-bitsequence becomes an I channel component. At the same time, a multiplier416 multiplies the 16-bit sequence by a 16-bit signal received from asignal generator 426, +1, −1, +1, −1, +1, −1, +1, −1, +1, −1, +1, −1,+1, −1, +1, −1 or −1, +1, −1, +1, −1, +1, −1, +1, −1, +1, −1, +1, −1,+1, −1, +1 and outputs the product as a Q component. Thus a complex ARMcode of length 16 is completely produced.

[0043] To generate an ARM code of length 64, the ARM code generator 200is extended to have two additional MUXes and two additional signalgenerators. In the same manner, three additional MUXes and three signalgenerators are required to the ARM code generator 200 to generate an ARMcode of 128. That is, to generate an I component and a Q component of anARM code of length 2^(n), (n−1) stages are required.

[0044] In the present invention, if an ARM code sequence of length 16 isrequired, one of 16 ARM code sequences listed below is selected usingthe ARM code generator 200. The 16 ARM code sequences of length 16 aregenerated by outputting four kinds of signals from the first MUX 400 forthe input of +1 & +1 and +1 & −1, eight kinds of signals from the secondMUX 402, and 16 ARM codes from the third MUX 404. The “i” in thefollowing ARM codes represents the imaginary or Q component. (ARM Codesof Length 16)  1 =  1 + 1i, 1 − 1i, 1 + 1i, −1 + 1i,  1 + 1i, 1 − 1i, −1− 1i, −1 + 1i,  1 + 1i, 1 − 1i, 1 + 1i, −1 + 1i, −1 − 1i, 1 − 1i, −1 −1i, −1 + 1i  2 =  1 − 1i, −1 − 1i, 1 − 1i, 1 + 1i,  1 − 1i, −1 − 1i,−1 + 1i, 1 + 1i,  1 − 1i, −1 − 1i, 1 − 1i, 1 + 1i, −1 + 1i, −1 − 1i,−1 + 1i, 1 + 1i  3 =  1 + 1i, 1 − 1i, −1 − 1i, −1 + 1i,  1 + 1i, 1 − 1i,1 + 1i, −1 + 1i,  1 + 1i, 1 − 1i, −1 − 1i, −1 + 1i, −1 − 1i, 1 − 1i, 1 +1i, −1 + 1i  4 =  1 − 1i, −1 − 1i, −1 + 1i, 1 + 1i,  1 − 1i, −1 − 1i, 1− 1i, 1 + 1i,  1 − 1i, −1 − 1i, −1 + 1i, 1 + 1i, −1 + 1i, −1 − 1i, 1 −1i, 1 + 1i  5 =  1 + 1i, 1 − 1i, 1 + 1i, −1 + 1i, −1 − 1i, 1 − 1i, −1 −1i, −1 + 1i,  1 + 1i, 1 − 1i, 1 + 1i, −1 + 1i,  1 + 1i, 1 − 1i, −1 − 1i,−1 + 1i  6 =  1 − 1i, −1 − 1i, 1 − 1i, 1 + 1i, −1 + 1i, −1 − 1i, −1 +1i, 1 + 1i,  1 − 1i, −1 − 1i, 1 − 1i, 1 + 1i,  1 − 1i, −1 − 1i, −1 + 1i,1 + 1i  7 =  1 + 1i, 1 − 1i, −1 − 1i, −1 + 1i, −1 − 1i, 1 − 1i, 1 + 1i,−1 + 1i,  1 + 1i, 1 − 1i, −1 − 1i, −1 + 1i,  1 + 1i, 1 − 1i, 1 + 1i,−1 + 1i  8 =  1 − 1i, −1 − 1i, −1 + 1i, 1 + 1i, −1 + 1i, −1 − 1i, 1 −1i, 1 + 1i,  1 − 1i, −1 − 1i, −1 + 1i, 1 + 1i,  1 − 1i, −1 − 1i, 1 − 1i,1 + 1i  9 =  1 + 1i, 1 − 1i, 1 + 1i, −1 + 1i,  1 + 1i, 1 − 1i, −1 − 1i,−1 + 1i, −1 − 1i, 1 − 1i, 1 + 1i, −1 + 1i, −1 − 1i, 1 − 1i, −1 − 1i,−1 + 1i 10 =  1 − 1i, −1 − 1i, 1 − 1i, 1 + 1i,  1 − 1i, −1 − 1i, −1 +1i, 1 + 1i, −1 + 1i, −1 − 1i, 1 − 1i, 1 + 1i, −1 + 1i, −1 − 1i, −1 + 1i,1 + 1i 11 =  1 + 1i, 1 − 1i, −1 − 1i, −1 + 1i  1 + 1i, 1 − 1i, 1 + 1i,−1 + 1i, −1 − 1i, 1 − 1i, −1 − 1i, −1 + 1i, −1 − 1i, 1 − 1i, 1 + 1i,−1 + 1i 12 =  1 − 1i, −1 − 1i, −1 + 1i, 1 + 1i,  1 − 1i, −1 − 1i, 1 −1i, 1 + 1i, −1 + 1i, −1 − 1i, −1 + 1i, 1 + 1i, −1 + 1i, −1 − 1i, 1 − 1i,1 + 1i 13 =  1 + 1i, 1 − 1i, 1 + 1i, −1 + 1i, −1 − 1i, 1 − 1i, −1 − 1i,−1 + 1i, −1 − 1i, 1 − 1i, 1 + 1i, −1 + 1i,  1 + 1i, 1 − 1i, −1 − 1i,−1 + 1i 14 =  1 − 1i, −1 − 1i, 1 − 1i, 1 + 1i, −1 + 1i, −1 − 1i, −1 +1i, 1 + 1i, −1 + 1i, −1 − 1i, 1 − 1i, 1 + 1i,  1 − 1i, −1 − 1i, −1 + 1i,1 + 1i 15 =  1 + 1i, 1 − 1i, −1 − 1i, −1 + 1i, −1 − 1i, 1 − 1i, 1 + 1i,−1 + 1i, −1 − 1i, 1 − 1i, −1 − 1i, −1 + 1i,  1 + 1i, 1 − 1i, 1 + 1i,−1 + 1i 16 =  1 − 1i, −1 − 1i, −1 + 1i, 1 + 1i, −1 + 1i, −1 − 1i, 1 −1i, 1 + 1i, −1 + 1i, −1 − 1i, −1 + 1i, 1 + 1i,  1 − 1i, −1 − 1i, 1 − 1i,1 + 1i

[0045] If an ARM code sequence of length 256 is required, one of 256 ARMcode sequences listed below is selected using the ARM code generator200.

[0046] As described before, the ARM code generator 200 can generate ARMcode sequences of different lengths. When a controller (not shown)applies a control signal corresponding to an ARM code of a desiredlength to the ARM code generator 200, the ARM code generator 200generates an ARM code of the desired length and feeds it to thesub-carrier selector 201.

[0047] Returning to FIG. 2, the sub-carrier selector 201 selectssub-carriers for three ARM code sequences of length 16, 256, and 128received from the ARM code generator 200 according to a desired sequenceof length 2^(n), that is, the characteristic of an intended preamble.The frequency mask 202 inserts null data for a DC component and guardintervals in the selected sub-carriers according to an IFFT mode in viewof the nature of OFDM. If 53 sub-carriers are selected, the frequencymask 202 generates 64 sub-carriers by inserting 11 null data. If 201sub-carriers are selected, the frequency mask 202 generates 256sub-carriers by inserting 55 null data. The IFFT 203inverse-fast-Fourier-transforms the output of the frequency mask 202 andoutputs a time-domain signal. The preamble assembler 204 concatenatesthe time-domain signal, that is, a short preamble sequence and a longpreamble sequence to thereby generate a downlink/uplink preamble or anSTC preamble.

[0048] Now the operation of the sub-carrier selector 201 and thefrequency mask 202 will be described referring to FIG. 3. Thesub-carrier selector 201 and the frequency mask 202 are implementedseparately as illustrated in FIG. 2. The sub-carrier selector 201 mapsthe samples of an ARM code generated from the ARM code generator 200 toOFDM sub-carriers. Then the frequency mask 202 deletes samples orinserts null data at predetermined sub-carrier positions for a DCcomponent and a guard interval in the OFDM sub-carriers. For clarity ofdescription, the following description of the sub-carrier selector 201and the frequency mask 202 is made with the appreciation that theyoperate integrally.

[0049]FIG. 3 schematically illustrates the operations of the sub-carrierselector 201 and the frequency mask 202. It is assumed that a pattern agenerator 304, a pattern b generator 305, and a pattern c generator 306perform the operations of the sub-carrier selector 201 and the frequencymask 202 according to the lengths of ARM codes (i.e., 16, 256, and 128,respectively), that is, select sub-carriers for the samples of the ARMcodes and mask frequencies. Referring to FIG. 3, upon receipt of an ARMcode sequence of length 16, the pattern a generator 304 selectssub-carriers for the samples of the ARM code of length 16 among given 64OFDM sub-carriers. That is, 16 sub-carriers are selected for the 16samples of the ARM code, a1 to a16. Then the pattern a generator 304removes the sub-carriers of 4 samples, a13 to a16 from the 16sub-carriers together with 7 sub-carriers related with the 4sub-carriers, and inserts null data in the remaining 41 sub-carriersexcluding the sub-carriers having the 12 samples. Thus the ARM code oflength 16 is patterned into {0, 0, a12, 0, 0, 0, a11, 0, 0, 0, a10, 0,0, 0, a9, 0, 0, 0, a8, 0, 0, 0, a7, 0, 0, 0, 0 (reference point), 0, 0,0, a1, 0, 0, 0, a2, 0, 0, 0, a3, 0, 0, 0, a4, 0, 0, 0, a5, 0, 0, 0, a6,0, 0}.

[0050] In the pattern the reference point 0 signifies a DC component inthe time domain. The pattern a generator 304 inserts null data in the 4sub-carriers of the deleted 4 samples and 7 sub-carriers interveningbetween the sub-carriers in order to define a guard interval withrespect to the reference point 0.

[0051] Briefly describing, the pattern a generator 304 generates Patterna for the input of an ARM code of length 16 and then outputs Pattern ato the IFFT 203.

[0052] Pattern a: {0, 0, a12, 0, 0, 0, a11, 0, 0, 0, a10, 0, 0, 0, a9,0, 0, 0, a8, 0, 0, 0, a7, 0, 0, 0, 0 (reference point), 0, 0, 0, a1, 0,0, 0, a2, 0, 0, 0, a3, 0, 0, 0, a4, 0, 0, 0, a5, 0, 0, 0, a6, 0, 0}

[0053] Upon receipt of an ARM code sequence of length 256, the pattern bgenerator 305 maps the 256 samples of the ARM code to 256 given OFDMsub-carriers. That is, the 256 samples of the ARM code, b1 to b256 aremapped to the 256 OFDM sub-carriers. Then the pattern b generator 305removes the sub-carriers of 56 samples, b201 to b256 among the 256sub-carriers. Thus the ARM code of length 256 is patterned into Patternb.

[0054] Upon receipt of an ARM code sequence of length 128, the pattern cgenerator 306 maps the 128 samples of the ARM code to 128 given OFDMsub-carriers. That is, the 128 samples of the ARM code, c1 to c128 aremapped to the 128 OFDM sub-carriers. Then the pattern c generator 306removes the sub-carriers of 26 samples, c103 to c128 among the 128sub-carriers. Null data is inserted in the remaining sub-carriersexcluding the 102 sub-carriers. Thus the ARM code of length 128 ispatterned into Pattern c.

[0055] Pattern c: {c102, 0, c101, 0, c100, 0, . . . c57, 0, c56, 0, c55,0, c54, 0, c53, c52, 0 (reference point), c1, c2, 0, c3, 0, c4, 0, c5, .. . 0, c49, 0, c50, 0, c51}, or {c102, c101, 0, c10, 0, c9, 0, . . .c57, 0, c56, 0, c55, 0, c54, 0, c53, 0, c52, 0 (reference point), c1, 0,c2, 0, c3, 0, c4, 0, c5, . . . 0, c49, 0, c50, c51}

[0056] Aside from the above two patterns, the pattern c generator 306can generate an ARM code sequence in

[0057] Pattern c: {c0, c100, 0, c99, 0 . . . c55, 0, c54, 0, c53, 0,c52, 0, c51, 0 (reference point), c1, 0, c2, 0, c3, 0, c4, 0, c5, . . .0, c48, 0, c49, 0, c51, 0} or {c100, 0, c99, 0, c98, 0, . . . c55, 0,c54, 0, c53, 0, c52, 0, c51, 0, 0 (reference point), 0, c1, 0, c2, 0,c3, 0, c4, 0, c5, . . . 0, c49, 0}

[0058] The structures of a downlink transmission frame and an uplinktransmission frame will be described with reference to FIG. 9.

[0059] The downlink/uplink transmission frame is the same in structureto the conventional frame. The downlink transmission frame uses twopreambles in concatenation, that is, a preamble 911 and a preamble 912.The preamble 911 is a short preamble produced by repeating an ARM codesequence of Pattern a of length 16 eight times and then inverting thesign of the ARM code sequence. That is, nine ARM code sequences ofPattern a and one sign-inverted ARM code sequence occur in the shortpreamble 911. The preamble 912 is a long preamble produced by repeatingan ARM code sequence of Pattern b of length 256 once. That is, two ARMcode sequences of Pattern b occur in the long preamble 912. A CP (CyclicPrefix) is a repetition of the last few bits of data following the CP toprevent multipath interference. The uplink transmission frame has a longpreamble 913 of length 256. The long preamble 913 is produced byrepeating the ARM code sequence of Pattern b once. When a transmitdiversity antenna is used, a data frame has an STC preamble 914 oflength 256.

[0060] Here, the downlink preamble is obtained by concatenating theshort preamble 911 and the long preamble 912. The short preamble 911 isobtained by inserting 11 nulls into a signal generated from the patterna generator 304, performing 65-point IFFT operation on the resultingsequence, repeating the IFFT output eight times, and inverting the signof the IFFT output. The long preamble 912 is obtained by inserting 55nulls into a signal generated from the pattern b generator 305,performing 256-point IFFT operation on the resulting sequence, andrepeating the IFFT output once.

[0061] Generation of the short preamble sequence will be described withreference to FIG. 6. FIG. 6 illustrates a short preamble sequencegeneration procedure in the MPP sequence generating apparatusillustrated in FIG. 2.

[0062] Referring to FIG. 6, the ARM code generator 200 generates an ARMcode sequence of length 16. The sub-carrier selector 201 selects 12samples among the 16 samples of the ARM code sequence and assigns themto 53 sub-carriers from #−26 to #26 by inserting nulls to the 12 samplesin Pattern a. The frequency mask 202 performs frequency-masking byinserting nulls to sub-carriers #27 to #37 input to the 64-point IFFT.FIG. 10 illustrates a frequency mask generator according to anotherembodiment of the present invention. As illustrated, the frequency maskfrequency-masks signals in the frequency domain to be applied to theinput of the IFFT 203.

[0063] The IFFT 203 performs 64-point IFFT on the output of thefrequency mask 202 and generates a preamble sequence 605 in the timedomain in which the pattern A occurs four times.

[0064] Generation of the long preamble sequence will be described withreference to FIG. 7. FIG. 7 illustrates a long preamble sequencegeneration procedure in the MPP sequence generating apparatusillustrated in FIG. 2.

[0065] Referring to FIG. 7, the ARM code generator 200 generates an ARMcode sequence of length 256. The sub-carrier selector 201 selects 200samples among the 256 samples of the ARM code sequence and assigns themto 201 sub-carriers from #−100 to #100 in Pattern b. The frequency mask202 performs frequency-masking by inserting nulls to sub-carriers #101to #155 input to the 256-point IFFT. FIG. 11 illustrates a frequencymask generator according to a third embodiment of the present invention.As illustrated, the frequency mask frequency-masks signals in thefrequency domain to be applied to the input of the 256-point IFFT. TheIFFT 203 performs 256-point IFFT on the output of the frequency mask 202and generates a preamble sequence 705 in the time domain in which thepattern B occurs twice.

[0066] Generation of an STC preamble sequence will be described withreference to FIG. 8. FIG. 8 illustrates an STC preamble sequencegeneration procedure in the MPP sequence generating apparatusillustrated in FIG. 2. Referring to FIG. 8, the ARM code generator 200generates an ARM code sequence of length 128. The sub-carrier selector201 selects 102 samples among the 128 samples of the ARM code sequenceand assigns them to 201 sub-carriers from #−100 to #100 in Pattern c.The frequency mask 202 performs frequency-masking by inserting nulls tosub-carriers #101 to #155 input to the 256-point IFFT. The IFFT 203performs 256-point IFFT on the output of the frequency mask 202 andgenerates an STC preamble sequence 805 in the time domain.

[0067] As described above, a BWS communication system selects adownlink/uplink preamble and an STC preamble having excellent PAPR andcorrelation characteristics. Distortion of signal output during RFtransmission caused by bad PAPR characteristics leads to decreasedsignal acquisition performance and makes synchronization acquisitiondifficult, thereby resulting in impossible communication. If thedownlink/uplink preamble and the STC preamble indicating the presence orabsence of data are not acquired, the data cannot be received. Iffrequency synchronization being a function offered by the preambles isnot acquired, tens of bits to hundreds of bits mapped to one symbol aredistorted and thus a whole data block is lost. These preambles use bursttechnology.

[0068] In general, a sequence of S(f) is called an aperiodic sequence inrelation to calculating its correlation. After IFFT, a signal S(t) isgood if it has a low PAPR and has a low cross correlation value. Thatis, if synchronization is acquired, a sequence having a low correlationvalue is good. If synchronization is acquired, a sequence having a highcorrelation value is good. However, an aperiodic sequence having anexcellent PAPR performance is not known in reality. Therefore, thepresent invention proposes an ARM code that is excellent as an aperiodicsequence for application to the downlink/uplink preamble and the STCpreamble. Even if the ARM code is shortened, that is, part of thesamples of the ARM code are lost, its PAPR performance after IFFT is notdecreased and its correlation value is high. Therefore, a preamblesequence from the ARM code is excellent in performance relative to anexisting preamble used in the conventional BWA/WLAN system.

[0069] In accordance with the present invention, downlink/uplink and STCpreamble sequences with a minimum PAPR are generated using ARM codes ina BWA communication system. Therefore, a synchronization acquisitionprobability is maximized and thus the overall system performance isimproved. Furthermore, preamble sequences of various lengths can begenerated in a relatively simple hardware.

[0070] While the invention has been shown and described with referenceto certain preferred embodiments thereof, it will be understood by thoseskilled in the art that various changes in form and details may be madetherein without departing from the spirit and scope of the invention asdefined by the appended claims.

What is claimed is:
 1. A preamble sequence generating method in a BWA(Broadband Wireless Access) communication system using OFDM (OrthogonalFrequency Division Multiplexing), comprising the steps of: generating atleast one ARM (Aperiodic Recursive Multiplex) code by outputting a thirdsequence by multiplying a first sequence having a first length and asecond sequence having the first length containing alternating +1s and−1s, outputting a fourth sequence by time-multiplexing the firstsequence and the third sequence, and resetting the fourth sequence asthe first sequence until an intended length of the ARM code is achieved;mapping the samples of the ARM code to sub-carriers equally distributedin a frequency band; and generating a preamble sequence by performingIFFT (Inverse Fast Fourier Transform) on sub-carriers containing ARMcode samples remaining from deleting a predetermined number of ARM codesamples from the ARM code samples, a reference component representing aDC component, and null components inserted between the remaining ARMcode samples and the reference component.
 2. The preamble sequencegenerating method of claim 1, wherein the predetermined ARM code samplesare deleted to prevent adjacent channel interference.
 3. The preamblesequence generating method of claim 1, wherein the DC component ismapped to a sub-carrier having a minimum frequency among thesub-carriers.
 4. A preamble sequence generating apparatus in a BWA(Broadband Wireless Access) communication system using OFDM (OrthogonalFrequency Division Multiplexing), comprising: an ARM (AperiodicRecursive Multiplex) code generator for generating at least one ARM codeby outputting a third sequence by multiplying a first sequence having afirst length and a second sequence having the first length containingalternating +1s and −1s, outputting a fourth sequence bytime-multiplexing the first sequence and the third sequence, andresetting the fourth sequence as the first sequence until an intendedlength of the ARM code is achieved; a sub-carrier selection andfrequency mask unit for mapping the samples of the ARM code tosub-carriers equally distributed in a frequency band; and an IFFT(Inverse Fast Fourier Transformer) for generating a preamble sequence byinverse-fast-Fourier-transforming sub-carriers containing ARM codesamples remaining from deleting a predetermined number of ARM codesamples from the ARM code samples, a reference component representing aDC component, and null components inserted between the remaining ARMcode samples and the reference component.
 5. The preamble sequencegenerating apparatus of claim 4, wherein the predetermined ARM codesamples are deleted to prevent adjacent channel interference.
 6. Thepreamble sequence generating apparatus of claim 4, wherein the DCcomponent is mapped to a sub-carrier having a minimum frequency amongthe sub-carriers.
 7. A preamble sequence generating apparatus in a BWA(Broadband Wireless Access) communication system using OFDM (OrthogonalFrequency Division Multiplexing), comprising: an ARM (AperiodicRecursive Multiplex) code generator for generating at least one ARM codeby outputting a third sequence by multiplying a first sequence having afirst length and a second sequence having the first length containingalternating +1s and −1s, outputting a fourth sequence bytime-multiplexing the first sequence and the third sequence, andresetting the fourth sequence as the first sequence until an intendedlength of the ARM code is achieved; a sub-carrier selection andfrequency mask unit for one-to-one mapping a second number of ARM codecomponents to a second number of sub-carriers among a third number ofsub-carriers equally distributed in a frequency band, the second numberbeing the number of the components of the ARM code, minus, a DCcomponent and a first number of ARM code components set to preventadjacent channel interference, and mapping null components tosub-carriers excluding from the second number of sub-carriers; and anIFFT (Inverse Fast Fourier Transformer) for generating a preamblesequence by inverse-fast-Fourier-transforming the sub-carriers receivedfrom the sub-carrier selection and frequency mask unit.
 8. The preamblesequence generating apparatus of claim 7, wherein the first number ofARM code components are mapped to outermost sub-carriers in thefrequency band.
 9. The preamble sequence generating apparatus of claim9, wherein the DC component is mapped to a sub-carrier having a minimumfrequency among the sub-carriers.
 10. The preamble sequence generatingapparatus of claim 7, further comprising a preamble assembler forgenerating a different preamble sequence by using preamble sequencesoutput from the IFFT individually or in combination.
 11. A preamblesequence generating method in a BWA (Broadband Wireless Access)communication system using OFDM (Orthogonal Frequency DivisionMultiplexing), comprising the steps of: generating at least one ARM(Aperiodic Recursive Multiplex) code by outputting a third sequence bymultiplying a first sequence having a first length and a second sequencehaving the first length containing alternating +1s and −1s, outputting afourth sequence by time-multiplexing the first sequence and the thirdsequence, and resetting the fourth sequence as the first sequence untilan intended length of the ARM code is achieved; one-to-one mapping asecond number of ARM code components to a second number of sub-carriersamong a third number of sub-carriers equally distributed in a frequencyband, the second number being the number of the components of the ARMcode, minus, a DC component and a first number of ARM code componentsset to prevent adjacent channel interference, and mapping nullcomponents to sub-carriers excluding from the second number ofsub-carriers; and generating a preamble sequence byinverse-fast-Fourier-transforming the sub-carriers mapped to the secondnumber of ARM code components and the null components.
 12. The preamblesequence generating method of claim 11, wherein the first number of ARMcode components are mapped to outermost sub-carriers in the frequencyband.
 13. The preamble sequence generating method of claim 11, whereinthe DC component is mapped to a sub-carrier having a minimum frequencyamong the sub-carriers.
 14. The preamble sequence generating method ofclaim 11, further comprising the step of generating a different preamblesequence by using preamble sequences output from the IFFT individuallyor in combination.